1. Scope of the Invention
The invention is in the field of optical devices for systems transporting and processing optical signals. In particular the invention concerns an optical 1.times.2 branching element provided with three optical ports, in which an optical signal injected via a first optical port is split into two signals of equal power that exit via a second and a third optical port respectively, and in which an optical signal injected via the second or the third port exits via the first optical port. For a branching element with an identical function, an application has also simultaneously been filed by applicant in the Netherlands (date of application: 24 Jul. 1996; application number: NL 1003669; title of the application: Optical non-linear branching element with MZ interferometer). The specification of said simultaneous application, hereinafter referred to as application P1, is deemed to be incorporated in the present application.
2. State of the Art
The splitting of optical signals is one of the most important basic functions in optical systems and networks. Optical signal splitters based on such channel-shaped waveguides as optical fibres or integrated waveguide structures are based on two fundamentally different physical principles. One type of signal splitter uses interference, e.g. the directional coupler and the splitter based on an MZ-interferometer. The second type of signal splitter uses symmetry, e.g. the symmetrical Y splitter and the asymmetric Y splitter dimensioned as mode filter.
For passive optical networks (PONs), used for both signal distribution and bidirectional traffic, and which have a tree-like branched structure, usually with a high degree of bifurcation, signal splitters are required on a large scale. For this purpose, (1.fwdarw.N) splitters are being designed which are mostly composed of (1.fwdarw.2) splitters of the above-mentioned types. In each (1.fwdarw.2) splitter, the optical signal is subject to a reduction in power of 3 dB in each of the two bifurcation directions. This is unavoidable (and natural) in the direction of further bifurcation (downwards), as the presented signal divides itself between the two possible bifurcation directions. Owing to time-reversal invariance (reciprocity) to which physical laws are subject, within the same structure this reduction also occurs for optical signals in the opposite direction (upwards), but this time as a real loss of signal. One way to avoid this loss in the upward direction is to let the wave-guiding structure in upward direction be different from that in downward direction. This can be achieved by switching the signals, e.g. with the use of externally, electrically or optically, controlled switches. However, this has the drawback that such a network no longer is passive, and in addition requires a complicated control system for the many switches.
Another option is the application of non-linear optical effects in the splitting structures indicated above, enabling a light signal itself to cause a switching effect to occur. G. J. M. Krijnen et al, "Simulation of low insertion loss non-linear Y junctions", Sensors & Actuators (Optical Transducers), Proceedings S & A symposium of the University of Twente, Enschede, The Netherlands, Nov. 15-16, 1990, University of Twente/Kluwer Technical Books, Deventer-Antwerpen, pp. 323-328 (hereinafter referred to as "Krijnen et al") discloses a simulation study of a symmetrical Y junction with a monomodal trunk and two monomodal branches, the branches of which at least are formed by identical waveguide sections in a non-linear optical medium. As a result of the symmetry, power splitting occurs in the bifurcation direction (downwards). In the upward direction, a light signal presented through either of the branches causes the index of refraction to increase, as a result of which the symmetry is broken. This causes the Y junction to become asymmetric and to act as a mode filter for the said signal: the light signal propagates fully as a zero-order mode signal in the trunk of the Y junction. In this connection, it is noted that in the above-mentioned types of unswitched (1.fwdarw.2) splitter, one part (half) is always converted into a first-order mode, said part radiating off upon entering the monomodal trunk of the Y junction, resulting in the loss of power mentioned. H. Fouckhardt and Y. Silberberg, "All-optical switching in waveguide X junctions", J. Opt. Soc. Am. B, Vol. 7, No. 5, May 1990, pp. 803-809 (hereinafter referred to as "Fouckhardt and Silberberg") disclose an asymmetric X junction in a non-linear medium that enables an optical signal to be switched using an optical control signal to be injected separately. Said control signal exhibits a switching effect (see more in particular FIG. 4(c) and FIG. 5(c) of Fouckhardt and Silberberg) which is similar to that described for the non-linear Y junction of Krijnen et al. A drawback of the splitting structures described in Fouckhardt and Silberberg, and Krijnen et al is that either they require extremely high optical powers, or relatively great non-linear optical effects are required, for which no suitable materials are available to date.
The entire contents of Krijnen et al, Fouckhardt and Silberberg, and U.S. Pat. No. 5,315,422 are incorporated herein by reference.